Materials Science Forum Vols. 546-549

Abstract: The hexagonal crystal structure of AZ31 results in a very high mechanical anisotropy and a poor formability of this alloy. In order to address these problems the influence of twinning, slip and dynamic recrystallization (DRX) on the microstructure and texture evolution during compression of AZ31 has been studied over a range of temperatures. Cylindrical samples were tested uniaxially in compression from room temperature to 350oC, with the compression axis parallel to either the normal direction (ND) or the transverse direction (TD) of the hot-rolled sheet from which the samples were cut. The microstructure was characterized using electron backscattered diffraction (EBSD) analysis. Extensive twinning was only observed in the TD samples at all temperatures. The effect of the difference in twinning and slip between the TD and ND samples on the mechanism of dynamic recrystallization has also been investigated. For this, the grains formed during DRX were identified by their appearance in EBSD orientation maps and the orientations of these grains determined.

Abstract: The effect of second phase on the mechanical properties of AZ61 and AZ91 Mg-Al-Zn magnesium alloy processed by equal-channel angular extrusion (ECAE) has been investigated. The results revealed that cracks initiated at and propagated through the α-Mg/β-Mg17Al12 interfaces and grain boundaries of Mg-Al-Zn alloys during tensile deformation. For AZ61, the elongation increased with the increase of ECAE pass number and the decrease of grain size. However, the elongation of AZ91 with more second phase particles decreased with the increase of ECAE pass number and the decrease of grain size.

Abstract: Microstructure evolution and mechanical properties of an AZ61 Mg alloy processed by cyclic extrusion compression were investigated. It is shown that CEC process may be applied successfully to AZ61 Mg alloys, and this leads to excellent grain refinement with grain size of ~0.8μm after 15 passes at 573K and to significant improvements in yield strength, ductility and hardness of AZ61 materials with slight compressive strength decreases. Dislocations were induced at initial stage of CEC process, and with increased deformation, tangled dislocations developed to form dislocation boundaries and subgrain boundaries, and then evolved into low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs) which was regarded as continuous dynamic recovery and recrystallization (CDRR).

Abstract: Effects of extrusion on mechanical properties and damping capacity of Mg-1.8wt.%Cu -0.5wt.%Mn (MCM1805) alloy have been investigated. Tensile tests and dynamic mechanical analyzer were respectively used to measure tensile properties and damping capacity at room temperature of as-cast and as-extruded MCM1805 alloy. The microstructure was studied using optical microscope, X-ray diffraction and scanning electron microscope with an energy dispersive X-ray spectrometer. Granato-Lücke model was used to explain the influences of extrusion on damping capacity of MCM1805 alloy. The results showed that extrusion dramatically decreases the grain size but has little influence on phase composition and solute atoms concentration of MCM1805 alloy, and the grain refinement was the dominant reason for the obvious increase of tensile properties and decrease of internal friction of MCM1805 alloy.

Abstract: The fractographs of Mg-9Gd-4Y-0.6Zr alloy specimens which were tensioned at different temperatures were investigated by optical and scanning electron microscopy, respectively. The results showed that different slip systems were activated at different temperatures, which were responsible for varied deformation mechanisms and fracture mechanisms. At 25 °C, the enabled slip systems were few and only the slip systems on basal plane were able to be activated, so cleavage fracture was observed. At −196°C
the number of enabled slip systems increased, prismatic or pyramidal slip maybe occurred, so that an obvious low-temperature plasticity was observed, while fracture mechanisms were mainly microvoid coalescence fracture with cleavage fracture in local areas. At 250 °C, 300 °C and 350 °C, the multisystem slips on the basal planes, the prismatic planes and the pyramidal planes were able to be activated, while fracture mechanisms were also microviod coalescence fracture. At 400 °C, recrystallization happened, grain-boundary sliding in new fine recrystallized grains made the plastic deformation easy, showing coarse-grain superplasticity phenomenon, and an intergranular shear fracture took place.

Abstract: The cold-compressed AZ31 magnesium alloys with different (2.5%, 5%, 7.5%, 10%, 12.5% and 15%) were annealed at different temperatures (573, 623 and 673K) for different time. With aid of the optical microscopy, and microhardness tester, the microstructural evolution during annealing at different temperature of the compressed AZ31 has been investigated. The microstructural characterized results were further related to the hardness test results and the calculated activation energy. The results showed that for the compressed AZ31, the recrystallization can occur on the samples with the strain just above 10% and the deformation twinning plays an important role on the nucleation and grain growth of the recrystallization. It was found that the twinning boundary was the location of the recrystallization nuclei and a lot of recrystallized grains with equiaxed shape were found along the twinning boundaries. The relationships among the nuclei orientations and the crystallographic orientations on both sides of the twining boundary have been statistically investigated. The effects of the compressed strain and the annealing temperature on annealing behavior were also discussed based on the experimental results.

Abstract: The deformability of cast AM60B Mg alloy is investigated by compressing AM60B cast ingots at elevated temperatures. The empirical results show that cast AM60B Mg alloy was brittle at room temperature and prone to cracking during compression, however at the temperatures ranging from 573 to 673K, excellent deformability is demonstrated with around 70% compression ratio. The compressive deformation constitutive equation for AM60B Mg alloy at elevated temperatures was established. The stress-strain relationships predicted at elevated temperatures show good agreement with experimental findings.

Abstract: In this paper, the basic formability of stamping of magnesium-alloy AZ31B sheet has been studied through experiments. The stamping formability of magnesium-alloy AZ31B sheet, such as the conical cup value, bending formability, deep drawing formability, formability of hole expanding, forming limit has been studied by simulating processing experiments. The formability of stamping supplies the basic reference data for the stamping processing. It is also found that the formability of stamping of magnesium-alloy AZ31B sheets is poor at room temperature and is excellent at intermediate and high temperatures.

Abstract: Uniaxial tensile test of a cross rolled magnesium alloy sheet was conducted under different temperatures and strain rates. The mechanical propriety of AZ31 magnesium alloy sheet was analyzed according to the true strain-stress curves. Then the non-thermal drawing process, during which the temperature of die, blankholder and blank is 200°C while the punch is kept at room temperature, was simulated by the thermo-mechanical coupled finite element method. The deformation behavior and the temperature change in the drawing process was investigated. Due to the heat conduction, there was non-uniform distribution of temperature along flange area, force transfer area and deformation area. Therefore the resistance of the force transfer area is enhanced and the warm formability of magnesium alloy sheet can be further improved. The thermo-mechanical coupled simulation provides a good guide for the development of non-isothermal drawing techniques.

Abstract: A process of warm deep drawing of magnesium alloy with variable blank holder force has been presented in this study. A hydraulic press that can realize adjustable blank holder forces was developed and its working principle and control system introduced. A warm deep drawing mould was fixed and warm deep drawing experiments are conducted on the hydraulic press. Different variation schemes of the blank holder force with the stroke of the punch was tested, and compared with experiment results. The experimental results show that adopting the variable blank holder force technique can remarkably improve the forming performance and decrease the reduction ratio of thickness from 15.21% to 12.35%. It is further demonstrated that the suitable blank holder force variation scheme is an inverted V curve with the punch stroke.